1. Field of the Invention
The present invention relates to a charge coupled device (CCD), more particularly to a CCD device capable of preventing transfer gate electrodes from being destroyed by static electricity.
2. Description of the Prior Art
In recent years, a CCD device has been applied to solid image pickup devices, signal delay devices, and analog memory devices and so on.
A CCD is essentially an analog shift register, in which a group of signal charges stored in a potential well can be successively transferred as a piece of information by controlling the potential in a silicon substrate with use of clock pulses applied to transfer gate electrodes.
Operation of a CCD has been disclosed, for example, in Japanese Laid-Open Publication No. 21334/77. FIG. 1 shows a prior art two-phase driving CCD.
In FIG. 1, the CCD includes a gate insulating film 102 provided on the surface of a silicon substrate 100, a plurality of first layer polysilicon gate electrodes 104 provided on the surface of the gate insulating film 102, and a plurality of second layer polysilicon gate electrodes 106. An impurity diffusion layer 108 is formed on the silicon substrate 100 under the second layer polysilicon gate electrodes 106. A potential difference .DELTA..psi. is produced in the silicon substrate 100 when the first and second polysilicon gate electrodes 104 and 106 are placed at the same potential.
The first polysilicon electrodes 104 and the second polysilicon electrodes 106 are connected with each other as a set of transfer gate electrodes 110. These sets of transfer gate electrodes 110 are alternately connected to clock signal lines 112 and 114 which are then connected to pin terminals 116 and 118 of an IC package. A signal charge 120 is transferred in a direction of the arrow A by applying clock pulses .phi., .phi. to the transfer gate electrode 110.
The CCD is generally a MOS structure so that the input impedance of the clock signal lines is very high. Thus, the gate oxide film is liable to be destroyed when a high electrical field is applied between the gate electrode and the substrate due to static electricity.
Consequently, generally for the CCD, a circuit 122 for preventing electrostatic breakdown thereof is connected between the clock signal lines 112 and 116, and between the clock signal terminals 114 and 118, as shown in FIG. 1. The protective circuit 122 decribed above is constructed of a clamping diode 126 and a current-limiting protective resistance 124. With a resistor R and the diode D both providing a small time constant in the protective ciruit, a high voltage pulse on the clock signal line is clamped by the diode D, thus avoiding direct application of the high voltage pulse to the gate electrode. Thus, the gate insulating film 102 can be prevented from being destroyed.
Generally, the maximum withstanding voltage against static electricity is made higher as the value of the protective resistance made higher. FIG. 4 (I) shows a maximum electrostatic withstanding voltage characteristic of the gate insulating film having thickness of about 500 .ANG..
However, when the capacity of the CCD is made larger, many of the transfer gate electrodes are connected to the clock signal lines, so that input capacity C is increased to for example several hundreds of several thousands of pF, allowing the time constant of the protective circuit to be increased, thereby giving rise to a problem in that the protective circuit can not sufficiently protect the gate insulating film against a high voltage pulse caused by static electricity.
It is possible in view of resolving the problem to improve gate withstanding voltage against static electricity by increasing a value of the protective resistor as shown in FIG. 4(I), but this causes the clock operating speed of the CCD to be reduced as shown in FIG. 4(II).